ABSTRACT
Though cell cultures of most species are homogeneous in nature, subtle differences at the subcellular level do exist among the population. In order to isolate cells with slight genetic differences within a huge population, we have to devise a method for selecting or screening the population with a selection agent. Sometimes such selection agents can also cause minor but specific mutations, thereby altering the genetic makeup of the cells. Plants regenerated from such mutant cells exhibit altered phenotypes. Among the most common selection agents that are used to generate diseaseresistant phenotypes are metabolites, such as phytotoxin, produced by the pathogen, crude culture filtrate, and rarely, the pathogen itself (Daub, 1984; 1986). A list of important work done in this field is summarized in Table 25.1.
